The demonstration followed one outside the embassy in the early hours, during which windows on the first floor were smashed and pink paint was thrown on the building.

Similar protests took place at Syrian embassies across Europe as news of the deaths spread, with demonstrators in Egypt, Greece and Germany forcing their way into the buildings and ransacking offices.

At least 5,500 people have died in Syria since pro-democracy campaigners took to the streets last year protesting against President Bashar Assad's regime.

Human rights activists said on Friday night that Syrian forces had used tanks and machine guns to kill at least 200 people and wound hundreds in Homs, in what appeared to be the bloodiest episode since the uprising began.

Protest organisers in London condemned the violence and stone throwing outside the embassy, but said it was a understandable reaction to people’s anger and upset.

Ahmad Hussain, 47, who said he was the leader of the Syrian revolution co-ordination in Britain, said: “Hundreds of people have been killed today but Syrian blood – nobody cares about it.

“We wanted to invade the embassy but unfortunately we were not able to.

“There has been a bit of trouble between us and the police, I know they have a job to do but they have to understand our situation.”

Ali Khalaf, 31, a security consultant who has lived in England for 12 years, said: “We tried to keep things peaceful as much as we could, but it was not easy. People are very upset and angry.”

Kinana Saffur, 27, a student, was at the protest in Belgrave Square with her two young children.

Mrs Saffur has aunts, uncles and a cousin Syria who she has not been able to get in contact with.

She said: “On the day that we are meant to be remembering the 1982 massacre (in Syria), another one occurs.

“Our message is simple, we want the ambassador to be kicked out but the government have not replied. People are just really, really angry.

“I would not have done it (thrown rocks) but I can understand why people did, people are very upset – this has been going on for years now.”

The protest group continued to sing and chant "free Syria" until around 6.15pm. In total, 12 men were arrested at the two

demonstrations.

In Egypt around 50 protesters tore down the gate to the Syrian embassy in Cairo, ransacking the building and setting fire to the ground floor.

Police in Berlin also said that around 20 people had forced their way into the Syrian embassy in the German capital, damaging offices and spraying slogans on to the walls.